The use of magnetic field measurements in prior art subterranean surveying techniques for determining the direction of the earth's magnetic field at a particular point is well known. Techniques are also well known for using magnetic field measurements to locate subterranean magnetic structures, such as a nearby cased borehole. These techniques are often used, for example, in well twinning applications in which one well (the twin well) is drilled in close proximity and often substantially parallel to another well (commonly referred to as a target well).
The magnetic techniques used to sense a target well may generally be divided into two main groups; (i) active ranging and (ii) passive ranging. In active ranging, the local subterranean environment is provided with an external magnetic field, for example, via a strong electromagnetic source in the target well. The properties of the external field are assumed to vary in a known manner with distance and direction from the source and thus in some applications may be used to determine the location of the target well. In contrast to active ranging, passive ranging techniques utilize a preexisting magnetic field emanating from magnetized components within the target borehole. In particular, conventional passive ranging techniques generally take advantage of remanent magnetization in the target well casing string. Such remanent magnetization is typically residual in the casing string because of magnetic particle inspection techniques that are commonly utilized to inspect the threaded ends of individual casing tubulars.
In co-pending U.S. patent application Ser. No. 11/301,762 to McElhinney, a technique is disclosed in which a predetermined magnetic pattern is deliberately imparted to a plurality of casing tubulars. These tubulars, thus magnetized, are coupled together and lowered into a target well to form a magnetized section of casing string typically including a plurality of longitudinally spaced pairs of opposing magnetic poles. Passive ranging measurements of the magnetic field may then be advantageously utilized to survey and guide drilling of a twin well relative to the target well. This well twinning technique may be used, for example, in steam assisted gravity drainage (SAGD) applications in which horizontal twin wells are drilled to recover heavy oil from tar sands.
McElhinney discloses the use of, for example, a single magnetizing coil to impart the predetermined magnetic pattern to each of the casing tubulars. As shown on FIG. 1, a hand-held magnetizing coil 65 having a central opening (not shown) is deployed about exemplary tubular 60. A direct electric current is passed through the windings in the coil 65 (the current traveling circumferentially about the tubular), which imparts a substantially permanent, strong, longitudinal magnetization to the tubular 60 in the vicinity of the coil 65. After some period of time (e.g., 5 to 15 seconds) the current is interrupted and the coil 65 moved longitudinally to another portion of the tubular 60 where the process is repeated. To impart a pair of opposing magnetic poles, McElhinney discloses reversing the direction of the current about coil 65 or alternatively redeploying the coil 65 about the tubular 60 such that the electric current flows in the opposite circumferential direction. In the above described prior art method, substantially any number of discrete magnetic zone's may be imparted to a casing tubular to form substantially any number of pairs of opposing magnetic poles.
A SAGD well twinning operation typically requires a large number of magnetized casing tubulars (for example, in the range of about 50 to about 100 magnetized tubulars per target well). It will be readily appreciated, that drilling even a moderate number of such twin wells can result in the need for literally thousands of magnetized casing tubulars. While the above described manual method for magnetizing casing tubulars has been successfully utilized, it is both time and labor intensive. It is also potentially dangerous given the size and weight of a typical casing tubular (e.g., on the order of about 40 feet in length and 1000 pounds or more in weight). Moreover, such a manual process has the potential to lead to significant differences in the imparted magnetization from tubular to tubular, especially given the sheer number of magnetized tubulars required for a typical SAGD operation. It will be appreciated that in order to achieve optimum passive ranging results (and therefore optimum placement of the twin wells), it is preferable that each tubular have an essentially identical magnetic pattern imparted thereto.
Therefore, there exists a need for an apparatus and method for magnetizing a large number of casing tubulars. In particular, a semi or fully automated apparatus and method that reduces handling requirements and includes quality control would be advantageous.